Theoretical study of the structure and stability of the ferriporphyrin dimer (Fe(III)C34H31N4O4)2

The electronic and geometric structures and dissociation energies of the isolated dimeric ferriporphyrin IX molecule (Fe(III)C34H31O4N4)2 and its ion and the corresponding monomers Fe(III)C34H31O4N4+ in the states with different multiplicity were calculated using the density functional theory (DFT) B3LYP method with the basis set Gen-1 = 6-311+G*(Fe) + 6-31G(C, H, N, O). The energetic characteristics were refined using the more extended basis set Gen-2 = 6-311+G*(Fe) + 6-31G*(C, H, N, O). The computation results are compared with the available X-ray diffraction data for a powder of β-hematin, whose lattice is composed of analogous blocks—(Fe(III)C34H31O4N4)2 dimmers—linked with one another by hydrogen bonds. For the neutral dimer molecule, the lowest-lying states were found to be quasi-degenerate high-and medium-spin states with multiplicities of 11, 9, and 7, while the quintet and triplet states are ∼0.4 eV higher. For the lowest-lying state, the calculated and experimental parameters of the ferriporphyrin ring are in good agreement with each other. For the peripheral propionate, methyl, and vinyl groups, the discrepancies are more significant (especially for their mutual orientations with respect to the porphyrin ring) and are, most likely, caused by the factors that are significant in solids and vanish in the isolated molecule. The energies of dissociation of the neutral dimer and its ion into monomers were estimated at ∼3.35 and ∼3.75 eV, respectively. The trends in the behavior of the structural characteristics of the ferriporphyrin rings upon dimerization and ionization and with a change in the multiplicity of electronic states are analyzed.